The functional vessels of human and animal bodies, such as blood vessels and ducts, occasionally weaken or even rupture. For example, in the aortic artery, the vascular wall can weaken or tear, resulting in dangerous conditions such as aneurysms and dissections. Upon further exposure to hemodynamic forces, such an aneurysm can rupture. In Western European and Australian men who are between 60 and 75 years of age, aortic aneurysms greater than 29 mm in diameter are found in 6.9% of the population, and those greater than 40 mm are present in 1.8% of the population.
One intervention for weakened, aneurysmal or ruptured vessels is the use of an endoluminal device or prosthesis, such as a stent graft, to provide some or all of the functionality of the original, healthy vessel and/or preserve any remaining vascular integrity by replacing a length of the existing vessel wall that contains the site of vessel weakness or failure. Stent grafts for endoluminal deployment are generally formed from a tube of a biocompatible material and one or more stents to maintain a lumen therethrough. Stent grafts effectively exclude the aneurysm by sealing both proximally and distally to the aneurysm, and shunting blood through its length. A device of this type can, for example, treat various arterial aneurysms, including those in the thoracic aorta, abdominal aorta, iliac, or hypogastric artery.
Two closely related aspects of stent graft function are sealing and fixation. The stent graft generally engages the wall of the lumen on both ends of the aneurysm or other defect, at proximal and distal regions referred to as landing or sealing zones. The sealing zones are typically near the termini of the stent grafts. The seal between the stent graft and the vascular wall is typically formed at these locations as a result of the circumferential apposition of the stent graft to the vascular wall. Apposition is typically maintained by the radial force exerted by stents fixed to the stent graft.
It is also desirable to fix, or anchor, the stent graft in place. For example, proximal fixation in the neck region of the aorta is often critical for long term durability of an endoluminal repair using a stent graft. Fixation of the stent graft in part depends on mechanical anchoring mechanisms. For example, in one anchoring mechanism the frictional forces between the stent graft and aortic wall may be generated by an interference fit created between the stent graft and aorta wall. The frictional forces may be supported by an underlying stent or stents. The practice of over-sizing a device for the lumen into which it is to be placed may also increase these frictional forces. Fixation may also be assisted by small hooks or barbs that extend from the stent graft and penetrate the arterial wall. In both cases, fixation is immediate and does not require long term biological interaction. In contrast, tissue encapsulation may also occur in some devices over a longer time frame. Exposed stainless steel stent struts and other parts of the stent graft may eventually become completely encapsulated by tissue growth, thereby assisting fixation.
One example of an endoluminal device, is a bifurcated stent graft, which is known for use in treating abdominal aortic aneurysms. The proximal end of the bifurcated stent graft defines a single lumen for placement within the aorta, while the distal end of the bifurcated stent graft defines a bifurcated region that encompasses two lumens. One such stent graft is disclosed in PCT application WO98/53761 and is useful for repair of abdominal aortic aneurysms. That application discloses a stent graft that includes a sleeve or tube of biocompatible graft material, such as woven polyester fabric or polytetrafluoroethylene (PTFE), defining a main lumen and two iliac limbs, where the graft material is impermeable to blood flow. The stent graft also includes several stents secured therealong. The stent graft is designed to span an aneurysm that extends along the aorta between the iliac and renal arteries.
In the WO98/53761 application, the fabric-covered portion of the single-lumen proximal end of the stent graft bears against the wall of the aorta above the aneurysm and distal to the renal arteries to seal off the aneurysm. Thin wire struts of a juxtarenal attachment stent or anchoring stent traverse the renal artery ostia without occluding them. Barbs on the anchoring stent then help to anchor the stent graft in place.
One bifurcated stent graft approved by the Food and Drug Administration (FDA) to treat aortic aneurysms is the ZENITH® AAA Endovascular Graft (Cook Incorporated, Bloomington, Ind.). The ZENITH® AAA Endovascular Graft is made up of three prosthetic modules: a bifurcated main body module and two leg modules. The main body is positioned in the aorta. The legs are positioned in the iliac arteries and connect to the main body. The stent graft thus extends from a section of the aorta, usually below the renal arteries, and into both iliac arteries. The graft material is made of a woven polyester fabric like that used in open surgical repair. Standard surgical suturing techniques are used to sew the graft material to a frame of stainless steel stents. These self-expanding stents provide support for the graft material.
In a conventional stent graft, the graft material must be extremely strong, wear resistant and substantially impermeable to liquids, such that it provides a barrier to blood flow. This combination of attributes generally necessitates a thick or bulky graft material. For example, one commonly utilized graft material possessing these attributes is tightly woven polyester, which has a thickness of about 0.2 to 0.3 mm. Adding to the bulk of a typical stent graft device are a plurality of sutures, which are typically employed to attach the graft material to the stent(s) of the stent graft device.
Unfortunately, in order to achieve the aspects of stent graft function described above, the combination of the stent, the graft material and the sutures results in a prosthesis with a bulk that can limit the compressibility of the prosthesis. Consequently, delivery of conventional stent grafts may require a larger than preferred diameter delivery sheath, making delivery of the stent graft via percutaneous entry into the femoral artery, for example, infeasible. In addition, the larger the diameter of the delivery sheath and the bulk of the enclosed stent graft may limit the flexibility during endoluminal placement.
Accordingly, the present invention seeks to overcome at least one of these disadvantages.